The semiconductor industry is developing chips which have ever smaller structures, and consequently an ever increasing density of components, to increase the computing speed of processors, the storage volume of memory elements and the power of capacitors, and also to lower the costs for the components.
The increasing miniaturization and integration of complex electronic circuits requires not only the integration of semiconducting components but also an integration of passive components, such as, for example, capacitors, coils, resistors, etc. The variety of application areas has had the effect that the microchips have to meet much more stringent requirements. Since the microchips are present in many everyday devices, such as, for example, computers, cell phones, GPS receivers, CD/DVD drives, cameras, pocket calculators, wristwatches, domestic appliances, cars, etc., both the active semiconductor elements and the passive semiconductor elements have to satisfy many different requirements.
The capacitors at the present time are produced by using inorganic dielectrics, for example, insulating metal oxides arranged between two electrodes, and are initially completed as individual pieces during the fabrication of printed circuit boards and then placed individually on the PCB and soldered by means of conventional construction and connecting techniques. However, these techniques require the use of complex and cost-intensive “pick-and-place” machines, with the effect of increasing production costs.
Microchips that are presently used are generally based on silicon as the semiconductor material. The production of the passive components and the integration of these components are still comparatively complex and expensive in spite of the advanced methods of production. In the case of some areas of use, these costs are not significant, since the memory units usually remain on the item for a considerable time or are used for goods which command a high price. There are, however, a whole series of applications in which goods that are relatively inexpensive are used and the attached microchips make up a significant proportion of the costs, with the result that the remaining components that would otherwise be used are ruled out of everyday use for reasons of cost.
A considerable cost reduction and time saving could be achieved, for example, by using RFID tags (Radio Frequency Identification Tags) in the retail sector. In the case of these applications, the price of an RFID tag for labeling products must not exceed that of a conventional barcode tag. Therefore, in this “low performance” area, the production costs must be fractions of a cent.
Furthermore, the microchips must have properties such as great robustness or low weight, to allow them to be processed without any problems, or else have great flexibility, to allow them to also be used on curved surfaces.
Attempts have been made to develop capacitors which can be integrated in the various substrates without the use of “pick-and-place” machines. For example, R. Ulrich and L. Shaper: IEEE Spectrum, July 2003, have proposed creating a capacitor including inorganic dielectrics and metal electrodes directly on or in the printed circuit boards in a multistage production process. This multilayer construction is intended to be used on various substrates, such as, for example, fixed PCB substrates, for example FR4 or flexible PCBs made of polyamide. The problem with this technology is the relatively low thermal stability of the PCB materials, since the depositing of high-quality inorganic dielectrics generally requires temperatures above approximately 400° C. The result is therefore a compromise in which a low quality and reliability of the dielectrics is deliberately accepted in order to permit integration in the PCBs.
A further possibility for the integration of capacitors on flexible substrates specifically for cost-driven RFID applications is described in D. Redinger et al.: Device Research Conference Digest 2003, 187–188. This approach is based on printing techniques in the case of which both the electrodes and the dielectric of the capacitor are deposited by using low-cost printing processes. Allowance is made for the low thermal budget of the flexible polymeric substrates by using organic polymer dielectrics, such as a polyamide with a maximum process temperature of 190° C. The problem with this solution is the large space requirement of capacitors produced in this way, since the creation of polymer layers with adequately good insulating properties generally requires layer thicknesses of several 100 nm. The capacitor proposed by Redinger et al. has a polyimide layer thickness of approximately 1 μm. However, the power of a capacitor depends on the layer thickness and can be represented by the following formula:C=ε·A/twhere C is the capacitance, ε is the dielectric constant, A is the surface area and t is the layer thickness of the dielectric. Consequently, a great layer thickness inevitably leads to a small capacitance per unit area. When polymer layers of approximately 1 μm are used, as described by Redinger et al., a large capacitance can be achieved only by increasing the surface area. For example, RFID transponders which are intended for the 13.56 MHz frequency band require a resonance capacitor of 0.4 nF, with the result that, if the layer thickness described by Redinger et al. is used, this capacitor would take up a surface area of approximately 25 mm2.